1. Field of the Invention
The present invention relates to a moisture sensor and a process for producing the same. More particularly, the invention relates to a moisture sensor which is useful for measuring the water content of various industrial process gases or recycle gases, for measuring the water content of high-pressure gases contained in cylinders, and for humidity measurements, and also to a process for producing the sensor.
2. Description of the Prior Art
Moisture sensors are already known which comprise a solid substrate of insulator, piezoelectric crystal, semiconductor or the like, having a moisture sensitive film formed on the substrate; a polyelectrolyte serving as the moisture sensitive substance.
Of these, the moisture sensor, wherein quartz crystal oscillator or like piezoelectric element is used as the substrate, determines the water content based on the variation of oscillation frequency of the piezoelectric element, serving as an indicator, which results from a change in the overal weight of the moisture sensitive film due to its hygroscopic action. Accordingly the sensor is capable of measuring water content . . . ppm lower than other types wherein variation of resistance or capacitance of the moisture sensitive film is used as an indicator. Such piezoelectric moisture sensors are usually produced by forming a pair of electrodes on the piezoelectric element and then coating the electrodes with a polyelectrolyte.
However, because the moisture sensitive film is formed by coating, the film is insufficietly adhered to the electrode surface and has poor durability. Further the sensor has low responsivity and stability when measuring very low water content in the ppm range. The moisture sensor further has difficulty in controlling the thickness of the moisture sensitive film.
A moisture sensor of the resistance type or capacitance type wherein a moisture sensitive film of polyelectrolyte is formed between a pair of electrodes on an insulator or semiconductor has similar problems with respect to the durability of the film, responsiveness, stability, etc.
We have already found that the above problems can be overcome by a piezoelectric moisture sensor which comprises a thin, high polymer film formed by plasma polymerization, having hydrophilic groups introduced into its surface layer, serving as the moisture sensitive film (U.S. patent application Ser. No. 516,959), now U.S. Pat. No. 4,567,725 granted Jan. 7, 1986.
The present invention, further developed from the above findings, provides a moisture sensor which comprises a moisture sensitive film consisting essentially of a plasma-polymerized high polymer layer and which has increased durability and improved responsiveness.